Digital systems typically need to constantly read and store digital data during operations. Therefore, memory units with the function of storing data are important elements in a digital system. Static random access memory (SRAM) cells and dynamic random access memory (DRAM) are important classes of volatile memory.
SRAMs are widely used where fast data access is needed such as the Level 1 cache in a microprocessor. Further, SRAMs have applications in mobile technologies such as cell phones and laptop computers where fast data access is desired. SRAMs typically occupy more chip area than DRAMs but provide the advantage of fast data access and simple peripheral circuitry. Unlike DRAMs, SRAMs consume much less standby power, thus making them attractive for mobile technologies where battery power is limited.
FIG. 1 illustrates a circuit diagram of a conventional SRAM column 100. Referring to FIG. 1, SRAM column 100 includes SRAM cells 102 and 104. SRAM cell 102 includes metal-oxide semiconductor field-effect transistors (MOSFETs) 106, 108, 110, and 112. Transistors 106 and 110 are p-channel enhancement-type MOSFETs (PMOSFETs). Transistors 108 and 112 are n-channel enhancement-type MOSFETs (NMOSFETs). The gates of transistors 106 and 108 are connected together at a memory node 114. The gates of transistors 110 and 112 are connected together at a memory node 116. The drains of transistors 110 and 112 are connected to memory node 114. Further, the drains of transistors 106 and 108 are connected to memory node 116. SRAM cell 104 is represented as a block and includes the same components and connections as SRAM cell 106.
Referring to FIG. 1, SRAM cell 102 can be accessed for reading or writing by raising the voltage of a word line 118, thus turning on NMOSFET access transistors 120 and 122. In this way, memory nodes 114 and 116 can be connected to bit lines 124 and 126, respectively. A sense amplifier, read buffers, and write buffers (represented by block 128) can be utilized for reading and writing data to SRAM cell 102 via bit lines 124 and 126.
FIGS. 2 and 3 illustrates waveform timing diagrams for control signals applied to SRAM column 100 of FIG. 1 for read and write operations, respectively. Referring to FIGS. 2 and 3, a precharge (PC) signal is a signal applied to transistors 130 and 132 for charging bit lines 126 and 124, respectively. A column-select (CS) signal is enabled and applied to column selectors 134 and 136 for selecting the column of SRAM column 100. After the PC signal is disabled, a word line (WL) signal is applied to transistors 120 and 122 for connecting nodes 114 and 116 to bit lines 124 and 126. As a result, the logic values at nodes 114 and 116, a differential voltage, appear on bit lines 124 and 126. The time required for the differential voltage signal to reach a certain certain predetermined value is Δt. A RdSa signal can be applied for turning sense amplifier 128 on. The data signal shown in FIG. 2 indicates when data is available for reading. The state signal shown in FIG. 3 indicates the logic state of the SRAM column.
Sizing transistors in an SRAM cell is important for minimizing area requirements. Minimization of the total cell area can be important because a typical SRAM has a large number of cells, typically on the order of 106 cells per SRAM array. The sizes of the transistors should be selected such that the read operation does not upset the data stored in the cell. At the same time, a write operation to the cell should result in a change of the logic state of the cell. The requirements for the design of an SRAM cell for read and write operations are conflicting. Making the cell more stable to prevent read upsets may result in making the write operation to that cell more difficult. Sizing the transistors to enable easy write operations results in making the cell more prone to read upsets.
Static power consumption is another concern with regard to SRAM cells. Gate oxide tunneling can have a substantial impact on static power consumption in SRAM cells. Static power consumption can be reduced by adding transistors. However, increasing the number of transistors will increase the area requirements of the cell. Thus, it is desired to reduce static power consumption without increasing the transistor count in SRAM cells. In addition, it is desired to reduce static power consumption with negligible impact on static noise margin and access times.
Accordingly, there is a need to provide SRAM cells having reduced size requirements and static power consumption. Further, there is a need for SRAM cells with improved noise margin and access times.